1. Field of the Invention
This invention relates to gas turbine engines, and more particularly to an abradable rub strip circumscribing an array of rotor blades in the compression section of such an engine.
2. Description of the Prior Art
A gas turbine engine has a compression section, a combustion section and a turbine section. The compression section has a rotor assembly. In the compression section, an engine case circumscribes the rotor assembly. The rotor assembly includes an array of rotor blades extending outwardly across the working medium flowpath into proximity with the outer case. An annular flowpath for working medium gases extends axially through the compression section between the rotor assembly and the engine case.
In modern engines, an abradable rub strip circumscribes the array of rotor blades and is attached to the engine case. The rub strip is formed of an easily abradable material such that, as the rotor blades move outwardly in response to operative conditions, a groove is worn in the abradable material. Even when all components reach steady-state positions, the relative position of the rotor blades and the abradable rub strip decrease leakage as compared with constructions that do not have abradable rub strips. Accordingly, use of an abradable rub strip improves compressor performance over constructions not having the strips. One construction having an abradable rub strip is illustrated in U.S. Pat. No. 3,843,278 to Torell entitled, "Abradable Seal Construction".
It is desirable in modern engines to make the engine cases and the rotor blades of a lightweight material having a density less than 0.2 pounds per cubic inch. An example of such a metal alloy is titanium. The use of titanium is of some concern because a rubbing contact between a titanium blade and a titanium case may result in ignition of the titanium case and subsequent burn through of the case. Because of the close proximity of the rotor blades to the outer case, the rotor blades may cut through the rub strip and rub against the outer case during abnormal operating conditions. Such a rubbing contact might unavoidably occur when the rotor assembly rotates eccentrically about its axis during a bearing failure or in response to foreign object damage to the blades.
In response to the concerns expressed above, scientists and engineers seek to develop an effective abradable rub strip which limits the depth of penetration of the abradable rub strip by the rotor blades when a rotor bearing fails or when there is foreign object damage to the rotor assembly.